This application pertains to the art of telemetry systems for warning of abnormal conditions and, more particularly, to wheel mounted telemetry systems for monitoring the conditions of pneumatic vehicular tires. The invention is particularly applicable to systems for monitoring automobile tires for low inflation or profile and will be described with particular reference thereto. It will be appreciated, however, that the invention has broader applications, such as monitoring tire conditions of all types of vehicles, monitoring the mechanical movement of rotating or reciprocating machinery parts, and the like. Further, the apparatus can monitor symptoms of a variety of abnormal tire conditions, including underinflation, overinflation, weakened sidewalls, and the like.
A variety of tire monitoring systems have heretofore been proposed. Many of these systems have included wheel mounted radio transmitters for transmitting AM or FM radio signals indicative of the abnormal conditon. A central receiver received the radio signals and produced a visual or audio signal to warn the driver of the abnormal condition. In some systems, the carrier frequency was amptitude or frequency modulated to enable differentiation from stray radio signals.
One of the problems with the prior tire condition sensing systems has been false signals. The receiver was subject to receiving AM and FM radio signals from various other sources, such as TV stations, radio stations, CB radios, and the like. Even using a different range of frequencies than the FCC assigns to other TV and radio broadcasts did not eliminate false signal problems. Various harmonics, echoes, and tones from these signals would cause false indications of abnormal tire conditions.
Another problem was false signals caused by road conditions. For example, potholes and rough roads deflect the tire profile. Many prior art tire sensing systems could not distinguish between an abnormally low tire profile caused by underinflation and an abnormally low tire profile caused by impacting a pothole or other roughness in the road surface. Brick, cobblestone, or other washboard road surfaces are especially hard to distinguish from underinflation.
Another problem with prior art structures has been the cost and reliability in meeting FCC regulations. The FCC has assigned a band of frequencies which may be used for this purpose. However, the FCC requirements limit the duration of broadcasts and the periodicity of broadcasts severly. To meet these FCC requirements, various clocks are employed. However, such clock systems were expensive, and in some instances were unreliable under the extreme temperature, centrifugal force, and impact conditions to which wheel-mounted sensors are subject.
The present invention comtemplates a new and improved apparatus which overcomes all of the above-referenced problems and others, yet provides a tire condition sensing system which is simple to construct, highly reliable, and low in cost.